Method and apparatus for separating a fraction rich in c2+ from liquefied natural gas

ABSTRACT

A process and apparatus for separating a fraction rich in C 2+  from liquefied natural gas (LNG) is disclosed. In an embodiment, partial evaporation of the liquefied natural gas occurs in a heat exchanger. The partially evaporated natural gas is separated into a first gas fraction rich in C 1  and a first liquid fraction rich in C 2+ . In a separation column, rectification separation of the first liquid fraction rich in C 2+  into a second gas fraction rich in C 1  and a second liquid fraction rich in C 2+  occurs. The first gas fraction rich in C 1  is re-liquefied in the heat exchanger. At least one sub-stream of the re-liquefied gas fraction rich in C 1  is fed as a reflux to the rectification separation.

This application claims the priority of International Application No. PCT/EP2005/013748, filed Dec. 21, 2005, and German Patent Document No. 10 2005 000 634.5, filed Jan. 3, 2005, the disclosures of which are expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and apparatus for separating a fraction rich in C₂₊ from liquefied natural gas (LNG).

Generic methods are provided, for example, when the calorific value of liquefied natural gas does not match the desired specifications of the pipeline network into which the natural gas is to be fed. In such a case, a component to lower the calorific value, for example nitrogen, is added to the liquefied natural gas or components which cause an increase in the calorific value are removed from the liquefied natural gas. In what follows the aforementioned second alternative will be explained in more detail.

From U.S. Pat. No. 5,114,451 a generic method for separating a fraction rich in C₂₊ or C₃₊ from LNG is known in which—in contrast to the methods implemented to date—a sub-stream of the gas fraction obtained in the rectification separation of the (liquefied) natural gas rich in C₁ is re-liquefied and added to the rectification separation as a reflux. The remaining stream of the gas fraction rich in C₁ is compressed to the desired output or pipeline pressure only after the separation of the sub-stream which forms the aforementioned reflux. The method described in U.S. Pat. No. 5,114,451 makes it possible to increase the ethanol yield to economically interesting values, but this is at the expense of using of at least one cost-intensive condenser.

A generic method for separating a fraction rich in C₂₊ from LNG is known from U.S. Pat. No. 3,420,068. In this method, the methane-rich gas fraction obtained in the rectification separation of the partially evaporated natural gas is not recondensed, with the consequence that only average ethane yields can be achieved.

The object of the present invention is to propose a generic method and apparatus for separating a fraction rich in C₂₊ from liquefied natural gas (LNG) which makes an increase in the yield of the fraction rich in C₂₊ possible while simultaneously reducing the investment and operating cost of the process, in particular dispensing with condensation of the methane-rich gas fraction.

To achieve the aforementioned object, a generic method for separating a fraction rich in C₂₊ from liquefied natural gas (LNG) is provided which has the following procedural steps:

-   a) partial evaporation of the liquefied natural gas, -   b) separation of the partially evaporated natural gas into a first     gas fraction rich in C₁ and a first liquid fraction rich in C₂₊, -   c) rectification separation of the first liquid fraction rich in C₂₊     into a second gas fraction rich in C₁ and a second liquid fraction     rich in C₂₊, -   d) re-liquefaction of the first gas fraction rich in C₁, and -   e) addition of at least one sub-stream of the re-liquefied gas     fraction rich in C₁ as a reflux to the rectification separation.

In contrast to the methods mentioned initially, the first gas fraction rich in C₁ obtained in the separation of the partially evaporated natural gas is re-liquefied and taken at least partially to the rectification separation as a reflux. The refrigeration required for the re-liquefaction of the first gas fraction rich in C₁ can be provided from the liquid stream of natural gas which is exposed to heating up to the boiling point at most. Since the gas fraction rich in C₁ given up to the rectification separation as a reflux has a comparatively low ethane content, it effects a backwashing of ethane and heavy components from the overhead of the rectification separations and thus the desired increase in the C₂₊ yield is achieved.

The method and apparatus in accordance with the invention therefore makes it possible to realize ethane yields of more than 90% using an economically sensible method. This value is within the range of the ethane yield that can be achieved with the costly method in accordance with U.S. Pat. No. 5,114,451 but is clearly higher than the value of a process in accordance with U.S. Pat. No. 3,420,068.

BRIEF DESCRIPTION OF THE DRAWINGS

The method and apparatus in accordance with the invention and additional embodiments of the same are explained in more detail in what follows using the embodiments shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, liquefied and supercooled natural gas which is pumped by means of pump P1 at a pressure of between 15 and 30 bar is taken to the heat exchanger E1 from a LNG container S through line 1. In the heat exchanger, the liquefied natural gas is heated countercurrent to the gas fraction 4 rich in C₁ which is to be cooled and re-liquefied to just before the boiling point. The interval from the boiling point is typically 5° C., at the most 20° C.

Subsequently the heated natural gas is taken through line 1′ to an additional heat exchanger E2 and heated and partially evaporated in the same countercurrent to the second gas fraction 8 rich in C₁ which will be discussed in what follows.

The partially evaporated natural gas stream is taken to a separator D1 through line 2. In the separator the partially evaporated natural gas stream is separated into a first gas fraction rich in C₁ which is drawn off at the head of the separator DI through line 4 and a first liquid fraction rich in C₂₊. The latter is taken through line 3 in which a pump is provided, the separation column T and thus to rectification separation.

As an alternative to the separator D1 shown in FIG. 1, a rectifier column can be provided in which the separation of the partially evaporated natural gas 2 is realized. In this case, the partially evaporated natural gas stream 2 would be fed into the sump of the rectifier column. The required column reflux can be realized either by means of a stream drawn from the yet to be described line 6 or from the yet to be described line 8 after the heat exchanger E2 and before the valve c. This embodiment of the method in accordance with the invention makes it possible to increase the C₂ yield substantially, which results in a reduction of losses to the heating gas.

In the separation column T, rectification separation of the previously described fluid fraction 3 rich in C₂₊ into a second fraction rich in C₁ takes place which is drawn off at the head of the separation column T through line 8 and a second fluid fraction rich in C₂₊. The separation column T can have plates and/or packaging materials.

The aforementioned second fluid fraction rich in C₂₊ is extracted through line 10, in which a control valve e is provided, from the sump of the separation column T and given off from the process as a NGL (Natural Gas Liquids) fraction and taken for further use. A sub-stream of this fraction is evaporated in the heat exchanger E3 and taken though line 11 as a reboiler stream to the separation column T.

In an advantageous way rectification separation T takes place at a higher pressure than the separation D1 of the partially evaporated natural gas 2 into a first gas fraction rich in C1 and a first liquid fraction 3 rich in C₂₊ . Here, in the separation or separator D1, a pressure of preferably between 15 and 25 bar and in rectification separation T a pressure preferably between 30 and 40 bar is realized. To overcome the pressure differential between separator DI and separation column T, a pump P3 must be provided in the line 3.

The first gas fraction rich in C₁ obtained in the separation of the partially evaporated stream of natural gas 2 which is drawn off from the head of the separator D1 through line 4, is re-liquefied in the heat exchanger E1 countercurrent to the stream of natural gas 1 to be heated, pumped by means of pump P2 up to the pressure obtaining in the separation column T and then through the line sections 5 and 6—where a control valve d is provided in the line section 6—to the rectification separation stage T, preferably in the head area, as a reflux.

The second gas fraction rich in C₁ obtained in rectification separation T is drawn off at the head of the separation column T through line 8 and countercurrent to the stream of natural gas 1′ to be heated, at least partially, preferably completely re-liquefied in the heat exchanger E2 and taken through line 8′, in which a control valve c is located, to a pump distillation container D2. Through line section 7, in which a control valve is also located, that sub-stream of the first re-liquefied fraction rich in C1 which is not given up to the separation column T is taken as a reflux to this container.

From the pump distillation container D2 the C₂₊ depleted LNG product fraction is pumped by means of the pump P4 up the desired delivery or pipeline pressure—this is usually between 60 and 150 bar—and taken out of the process through line 9 in which a control valve a is similarly provided.

To the extent that the second liquid fraction rich in C₂₊ obtained in the rectification separation stage which is drawn off from the sump of the separation column T through line 10 is to be subjected to a C₂/C₃ separation, the procedure is preferably as shown in FIG. 2.

In this, the second liquid fraction rich in C₂₊ undergoes a (second) C₂/C₃ rectification separation in the separation column T′. From the sump of the separation column an LNG product fraction rich in C₃₊ is drawn off through line 18 and taken for further processing or use. A sub-stream of this fraction is partially evaporated in heat exchanger E5 and taken through line 19 as a reboiler stream to the separation column T′.

At the head of the separation column T′, a gas fraction rich in C₂/C₃ is drawn of through line 12, at least partially condensed in the lateral reboiler E4 which is connected to the separation column T through the lines 13 and then taken through line 14 to the pump distillation container D3.

The condensed fraction rich in C₂/C₃ is taken from the pump distillation container D3 to pump P5 through line 15 and pumped up to the desired delivery pressure. A sub-stream of the pumped fraction is given up as a reflux to the head of the separation column T′ through line 16 and control valve f while the primary stream of the pumped fraction is removed from the process through line 17 and taken for further use or processing.

Other C₂/C₃ separation processes can be realized as alternatives to the method described in FIG. 2, for example, the extraction of a gaseous overhead product fraction from the container D3.

Refining the method in accordance with the invention for separating a fraction rich in C₂₊ from liquefied natural gas, it is provided that the cooling of process streams which result from the separation T′ of the second liquid fraction rich in C₂₊ takes place countercurrent to the first fraction rich in C₂₊ and/or countercurrent to at least one liquid fraction which is drawn off from the rectification separation stage below the feed of the first liquid fraction rich in C₂₊.

In particular when the second liquid fraction 10 rich in C₂₊ is subjected to a C₂/C₃ rectification separation T′, cooling the gas fraction 12 rich in C₂/C₃ obtained in the C₂/C₃ rectification separation is recommended preferably in an exchange of heat countercurrent to the first liquid fraction 3 rich in C₂₊ preferably to a pressure-free storage temperature.

The heat exchanger required for this can be provided, for example, between pump P3 and the separation column T. So that pressure-free storage can be realized, a temperature of about −100° C. is necessary in the case of a gas fraction consisting primarily of ethane which is obtained at the head of the C₂/C₃ rectification separation T′. It is advantageous in this method that in the entire process outside refrigeration—cooling below ambient temperature—or the thermodynamic equivalent—condensation can be dispensed with.

If, as explained from the methods and apparatus shown in FIGS. 1 and 2, all process streams enter in liquid form (stream 1) and are given off again in liquid form (streams 9 and 10, or 9, 17 and 18) all cooling ultimately takes place from the supercooling of the LNG used.

The method and apparatus in accordance with the invention for separating a fraction rich in C₂₊ from liquefied natural gas (LNG) makes possible high yields of ethane while at the same time avoiding the use of cost-intensive condensers. 

1-10. (canceled)
 11. A process for separating a fraction rich in C₂₊ from liquefied natural gas (LNG), comprising the steps of: a) partial evaporation of the liquefied natural gas; b) separation of the partially evaporated natural gas into a first gas fraction rich in C₁ and a first liquid fraction rich in C₂₊; c) rectification separation of the first liquid fraction rich in C₂₊ into a second gas fraction rich in C₁ and a second liquid fraction rich in C₂₊; d) re-liquefaction of the first gas fraction rich in C₁; and e) feeding of a least one sub-stream of the re-liquefied gas fraction rich in C₁ as a reflux to the rectification separation.
 12. The method according to claim 11, wherein the re-liquefaction of the first gas fraction rich in C₁ takes place in an exchange of heat with the liquefied natural gas.
 13. The method according to claim 11, wherein the rectification separation takes place at a higher pressure than the separation of the partially evaporated natural gas into the first gas fraction rich in C₁ and the first liquid fraction rich in C₂₊.
 14. The method according to claim 13, wherein the separation of the partially evaporated natural gas into the first gas fraction rich in C₁ and the first liquid fraction rich in C₂₊ takes place in a pressure range between 15 and 25 bar and the rectification separation takes place in a pressure range between 30 and 40 bar.
 15. The method according to claim 11, wherein a sub-stream of the re-liquefied gas fraction rich in C₁ which is not taken as the reflux to the rectification separation is combined with the second gas fraction rich in C₁ obtained in the rectification separation.
 16. The method according to claim 11, wherein the second gas fraction rich in C₁ obtained in the rectification separation is at least partially re-liquefied by an exchange of heat with the liquefied natural gas.
 17. The method according to claim 11, wherein a cooling of process streams which result from a separation of the second liquid fraction rich in C₂₊ takes place countercurrent to the first liquid fraction rich in C₂₊ and/or countercurrent to at least one fluid fraction which is drawn off from the rectification separation below a feed of the first fluid fraction rich in C₂₊.
 18. The method according to claim 11, wherein the second liquid fraction rich in C₂₊ undergoes C₂/C₃ rectification separation, wherein a head condensation of the C₂/C₃ rectification separation takes place in a lateral boiler countercurrent to at least one liquid fraction which is drawn off from the rectification separation below a feed for the first fluid fraction rich in C₂₊,
 19. The method according to claim 11, wherein the second liquid fraction rich in C₂₊ undergoes C₂/C₃ rectification separation, wherein a gas fraction rich in C₂/C₃ obtained in the C₂/C₃ rectification separation is cooled in an exchange of heat countercurrent to the first liquid fraction rich in C₂₊ and cooled to a pressure-free storage temperature.
 20. The method according to claim 11, wherein the separation of the partially evaporated natural gas into the first gas fraction rich in C₁ and the first liquid fraction rich in C₂₊ takes place in a separator or a rectifier column.
 21. A process for separating a fraction rich in C₂₊ from liquefied natural gas (LNG), comprising the steps of: partial evaporation of the liquefied natural gas in a heat exchanger; separation of the partially evaporated natural gas into a first gas fraction rich in C₁ and a first liquid fraction rich in C₂₊; rectification separation of the first liquid fraction rich in C₂₊ into a second gas fraction rich in C₁ and a second liquid fraction rich in C₂₊; re-liquefaction of the first gas fraction rich in C₁ in the heat exchanger; and feeding of a sub-stream of the re-liquefied gas fraction rich in C₁ as a reflux to the rectification separation process.
 22. The process according to claim 21, wherein the step of re-liquefaction of the first gas fraction rich in C₁ in the heat exchanger includes the step of refrigeration of the first gas fraction rich in C₁ by the liquefied natural gas which is heated up to a boiling point.
 23. The process according to claim 21, wherein the first gas fraction rich in C₁ is re-liquefied in the heat exchanger countercurrent to a stream of the liquefied natural gas.
 24. An apparatus for separating a fraction rich in C₂₊ from liquefied natural gas (LNG), comprising: a heat exchanger, wherein the liquefied natural gas is partially evaporated in the heat exchanger; a separator coupled to the heat exchanger, wherein the partially evaporated natural gas is separated in the separator into a first gas fraction rich in C₁ and a first liquid fraction rich in C₂₊; and a separator column coupled to the separator, wherein rectification separation of the first liquid fraction rich in C₂₊ into a second gas fraction rich in C₁ and a second liquid fraction rich in C₂₊ occurs in the separator column; wherein the first gas fraction rich in C₁ is re-liquefied in the heat exchanger and wherein a sub-stream of the re-liquefied gas fraction rich in C₁ is fed as a reflux to the separation column.
 25. The apparatus according to claim 24, wherein the re-liquefaction of the first gas fraction rich in C₁ in the heat exchanger includes refrigeration of the first gas fraction rich in C₁ by the liquefied natural gas which is heated up to a boiling point.
 26. The apparatus according to claim 24, wherein the first gas fraction rich in C₁ is re-liquefied in the heat exchanger countercurrent to a stream of the liquefied natural gas. 